4 Answers
4

As the person who made that claim, the reason is that allegedly some greases can attack the epoxies found in some carbon fiber applications, causing a breakdown of the CF structure, and causing expansion which will jam the post in place.

The epoxy will otherwise not corrode, so it's not necessary for that purpose. The manufacturers also recommend you do not do it.

I have a carbon fiber seatpost in a steel frame that I greased a long time ago before I learned about this and it isn't stuck, but I might just be lucky.

And of course, if you use a grease specifically designed for carbon fiber applications it negates all these claims, for they refer to the run of the mill stuff.

Actually, this is a myth. Carbon parts will cause aluminum to oxidize, as a chemical reaction. "Carbon grease" is not actually grease. It's a friction compound which increases the friction between your fancy carbon seat post and your frame. Increasing the friction allows a lower torque on the fixing bolts for the individual part, which reduces the risk of crushing the part, or having it slip in the frame. Slippage of seat posts and handlebars in the frame was why we were originally told not to grease carbon parts. The grease reduced friction, which meant higher torque and more broken parts.
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zenbike♦Jun 20 '11 at 12:28

Carbon parts will cause aluminum to oxidize, as a chemical reaction which is why seat posts wind up stuck in frames. But that isn't why this is necessary.

"Carbon grease" is not actually grease. It's a friction compound which increases the friction between your fancy carbon seat post and your frame. Increasing the friction allows a lower torque on the fixing bolts for the individual part, which reduces the risk of crushing the part, or having it slip in the frame.

Slippage of seat posts and handlebars in the frame was why we were originally told not to grease carbon parts. The grease reduced friction, which meant higher torque and more broken parts.

After parts started to get stuck, they realized that they need something, but it had to increase friction, rather than decrease it. Hence, carbon "grease" compounds.

You can get carbon seat post specific grease, it's designed to reduce the clamping pressure that is needed to hold the seat post in place and prevent it either sticking or slipping. Pace make some (Pace RC005 Carbon Fibre Seatpost Grease) and I'm sure some other people do as well.

And a carbon-friendly grease may be a solution to the question which originally started this discussion, about keeping the inside of the frame's seat-tube rust-free. Moisture can become trapped between carbon and the frame as well as any other seat-post and the frame... good to know that such a product exists.
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DC_CARRDec 14 '10 at 20:13

There are actually fairly common issues with exactly that kind of permanent bonding. See answers above.
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zenbike♦Jun 23 '11 at 14:30

Interesting. Can't say I've ever seen it happen. I live in a fairly wet climate, maybe it only happens in drier places? Some other factor on why it would not happen to anyone that I know?
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Brian KnoblauchJun 23 '11 at 17:47

No idea, really. We live in a coastal area, with high heat and humidity (Dubai), but before here, I lived in Seattle, which is cool and consistently wet. Perhaps the salt in the air? But we've had consistent and similar issues in both places. It does tend to happen when the post is left in place for an excessively long period of time, without maintenance.
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zenbike♦Jun 25 '11 at 7:35